1. Field of the Invention
The invention relates to a process for the thermomechanical treatment of steel for torsionally-strained spring elements, the initial material being heated with a heating rate of at least 50 K/s and austenitized, and the product then being formed in at least one forming step with the formed product being quenched to below the martensite temperature to martensite and then tempered.
2. Description of Related Art
A process of the initially mentioned type is already known from German Patent No. DE 43 40 568 C2. In this known process, wire, as the initial material, is heated at a rate between 85 K/s and 100 K/s to a temperature above Ac3, specifically to a temperature of 860xc2x0 C. and then held for 10 to 30 seconds, in order, on the one hand, to achieve complete conversion of the ferrite structure into austenite, and on the other hand, to produce a uniform distribution of the carbon in the austenite. To achieve a higher level of fine graininess, then, forming takes place at 860xc2x0 C., the wire being ovalized in a first pass, rolled round in a second pass and then driven through a calibration nozzle. Afterwards, the wire is quenched and tempered again. The austenite is quenched to martensite in this known process with a microstructure which is not recrystallized. The strength or toughness properties of the wire produced using this known process can be improved with respect to increased vibration strength which is necessary for torsionally-strained spring elements.
German Patent No. DE 195 46 204 C1 discloses a process for producing spring elements from a heat treated steel in which thermomechanical treatment of the initial material is performed with the following steps:
a) The initial material of the steel is solutionized in the austenite range at temperatures from 1050 to 1200xc2x0 C.,
b) directly thereafter the first forming is performed above the recrystallization temperature,
c) directly thereafter a second forming is performed below the recrystallization temperature, but above Ar3.
d) further forming and working processes are carried out below the recrystallization temperature, but above the Ar3 temperature, the holding time lasting one minute,
e) then, the rolled product is quenched to below the martensite temperature and finally tempered.
The austenite is quenched in this known process to martensite, likewise, with a microstructure which is not recrystallized by the forming according to the aforementioned steps c) and d). Since, in this known process, forming is performed not only below the recrystallization temperature, but holding times are even longer before quenching at high temperatures, there is otherwise considerable grain growth.
According to research by the applicant, the use of the known process leads to fine acicular, but highly textured martensite microstructures with strength and toughness properties which are good in the rolling direction. The known process is therefore suited for spring elements which are exposed to tensile/compressive loading in the rolling direction, as is especially the case in leaf springs.
In torsion-strained spring elements, such as helical springs and stabilizers, the direction of maximum loading does not correspond to the preferred direction of maximum strength which is produced by the aforementioned process. Rather, thermomechanical treatment which leads to an unrecrystallized, less recovered austenite grain is not suited for steels for torsionally loaded spring elements, and it does not lead to an improvement of vibration strength.
The primary object of this invention is, therefore, to provide a process for thermomechanical treatment of steel in torsionally strained spring elements which leads to improvement of the strength or toughness properties of the spring steel in the direction of strain of the torsionally strained spring elements so that considerable recovery of vibration strength occurs.
This object is achieved in accordance with the present invention in a process for thermomechanical treatment of steel in torsionally strained spring elements essentially by the fact that the initial material is heated to a temperature above the recrystallization temperature and then is formed at such a temperature, that dynamic and/or static recrystallization of the austenite occurs, and that the austenite of the forming product recrystallized in this way is quenched.
In the process according to the invention, heating proceeds into the austenite range to above the recrystallization temperature in a very short time; this does not allow the austenite grains time to grow into coarser grains. Subsequent forming in the corresponding temperature range yields dynamic recrystallization (during forming) and/or static recrystallization (after forming); this is also called crystalline modification, and as a result, leads to extremely fine-grained austenite crystals. These ultrafine recrystallized crystallites are then converted, during subsequent quenching, into an extremely fine-acicular martensitic microstructure. After quenching treatment, finally, tempering to the desired strength-toughness combination takes place.
A key difference between the invention and the prior art lies in allowing the austenite to recrystallize, subjecting it to forming treatment in the recrystallized state, then allowing static and/or dynamic recrystallization to proceed, and finally quenching the recrystallized austenite to martensite.
The martensite formed by the process of the present invention, as compared to the microstructures which have been produced using the known processes, has highly improved strength and toughness properties in the strain direction of the torsionally strained spring elements so that the increase of strength is considerable.
Preferably, the initial material is heated with a heating rate between 80 and 150 K/s to a temperature of at least 900xc2x0 C., preferably a temperature between 900xc2x0 C. and 1200xc2x0 C. This heating takes place preferably inductively.
One especially good result is achieved when forming takes place in at least two forming steps above the recrystallization temperature. Also, several forming steps can be carried out above the recrystallization temperature, preferably four forming steps. Otherwise, it is recommended that forming be carried out with a total logarithmic degree of forming of at least 0.1.
The initially fine austenite crystallites are made even finer by the above described repeated static and dynamic recrystallization during or after forming.
To give no time for the austenite crystallites to grow between the individual forming steps, it is furthermore provided that the holding time between the forming steps be very short, in any case less than one minute. Preferably, only a few seconds of holding time are provided between the forming steps. Forming itself proceeds, in one preferred embodiment, in the temperature range between roughly 1000xc2x0 C. and 800xc2x0 C., and the material should be reheated between successive forming steps to enable recrystallization.
To even further refine the martensite microstructure which has formed after quenching, it is, moreover, provided that the material be re-austenitized quickly and the resulting austenite in turn requenched after further forming, or even without forming. Cold working before or after tempering is easily possible.
In the process in accordance with the invention, as the initial material, especially a silicon-chromium steel with a carbon content of 0.35% to 0.75% is used which is microalloyed with vanadium or another alloying element.
These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, show only a single embodiment in accordance with the present invention.